gh-137337: Clarify import statement namespace binding

[kovan]

Summary

The import statement documentation says names are defined "in the local namespace," which is inaccurate when the imported name has been declared global or nonlocal:

x = None
def f():
    global x
    import os as x  # Assigns to global namespace, not local

Updates two places in Doc/reference/simple_stmts.rst:

• The basic import description (step 2): "local namespace" -> "current namespace... just as an assignment statement would, including global, local, and nonlocal semantics"
• The from ... import description (step 2.d): "local namespace" -> "current namespace"

This follows the wording proposed by @nedbat and discussed with @serhiy-storchaka and @terryjreedy in the issue thread.

Test plan

• make -C Doc check passes
• make -C Doc html passes (no warnings)

🤖 Generated with Claude Code

• Issue: Documentation of import statement should include binding to global and nonlocal namespaces #137337

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